Magnetic recording media generally include a binder dispersion layer comprising a binder and one or more pigments overlying a substrate, wherein the pigments are dispersed within the binder. Typically, the pigments are magnetizable pigments comprising small, magnetizable particles. In some instances, the medium may be in the form of a composite having both back-coat and front-coat binder dispersion layers, although the pigment in the back-coat may or may not be a magnetizable pigment.
It has become desirable to have as high a loading of magnetizable pigment in the magnetic recording media as is reasonably possible. It is often preferred to have a binder dispersion comprising from about 70% to 85% by weight magnetizable pigment relative to the binder with as many magnetizable particles per unit area or unit volume as possible. It is also preferred to have a binder dispersion in which the magnetizable pigment comprises a plurality of small particles having a relatively high specific surface area. Higher pigment loading has the potential to provide high density magnetic recording media capable of storing more information.
Problems, however, remain in the art concerning magnetic recording media having a relatively high loading of very high surface area magnetizable pigment. To begin with, magnetizable pigments tend to agglomerate, and they are difficult to properly and fully disperse within the binder. Wetting agents, or dispersants, are often employed to facilitate such dispersion. For higher pigment loading, that is, the use of greater amounts by weight and number of magnetizable particles, greater amounts of such dispersants are required, which is not always desirable.
There are a number of reasons for using as little dispersant as possible. Dispersants tend to soften binder systems, decrease their toughness, and adversely affect their cure. Binders without dispersants can be more readily and reproducibly prepared. Further, excess dispersant may bloom from a cured binder system over time, leading to contamination of a recording head or the like, causing a change in the physical or chemical characteristics of the media.
To help alleviate these problems with added dispersants, polymeric dispersant binder compositions having chemically bound dispersants have been developed. Such compositions comprise polymers with polar functional moieties pendant from the polymer backbone that help disperse pigments. As a result of using these compositions, less dispersant or, in some instances, no dispersant is needed for dispersion of magnetic pigment in the binder. Commercially available polymeric dispersants for magnetic recording media applications typically contain a relatively low quantity of polar functional moieties, which are usually carboxyl or sulfonate groups present at levels less than about 0.1 milliequivalents of carboxyl or sulfonate group per gram or polymer. A higher degree of pigment-polymer interaction is often desired for purposes of dispersion and coatings stability, particularly in the case of pigments exhibiting small particle size, such as iron metal particles and barium ferrite.
Polyurethanes containing high levels of carboxylic acid functionality are known. U.S. Pat. Nos. 4,983,491 and 4,898,803 to Fuji describe polyurethanes said to be useful in a photolithographic application which are the reaction product of a diol having a carboxyl group with a diisocyanate. These materials have a carboxylic acid content of greater than 1.0 meq/g. Their use of these polyurethanes is unrelated to dispersion formation or magnetic recording. Graft carboxyl polyurethanes are not taught.
U.S. Pat. No. 4,571,364 (to Fuji Photo Film Co.) discloses polyurethane resins for magnetic binders in which polar groups including carboxyl, may be incorporated into the polymer. The claimed polar group content is greater than 0.1 meq/q. The sole example of a polyurethane containing carboxyl groups is Example 3. Example 3 is described in a referenced Japanese Patent Publication (No. 38760/78) to contain a water based polyurethane which is said to have 0.5 meq/g carboxyl groups. U.S. Pat. No. 4,788,103 to Fuji Photo Film Co. describes polyurethanes used as magnetic pigment binders having 0.03 to 2 weight percent carboxyl group (0.007 to 0.4 meq/g). Neither combinations with quaternary ammonium compounds nor graft side chains are taught.
U.S. Pat. Nos. 4,529,661 and 4,613,545 to Sony Corp. describe binders, including polyurethane binders, having polar groups, including carboxyl groups, at levels of from 200 to 50,000 grams of polymer per mole of polar group (0.02 to 5 meq/g). There are no examples of carboxyl containing polyurethanes in these patents.
U.S. Pat. No. 5,165,999 to Fuji Photo Film describes polyurethanes prepared from amino carboxylic acid diols used in magnetic recording media. Blending with a vinyl chloride type resin containing a polar group including quaternary ammonium polar group is also described. The claims are expressed in terms such that a carboxyl content range cannot be calculated. The level of carboxylic acid functionality in the examples is about to 0.07 meq/g (about 14,000 grams polymer per mole of carboxyl group).
Copending U.S. patent application Ser. No. 08/054,511 and U.S. patent application Ser. No. 08/054,312, assigned to the assignee of the present invention, describes the use of polyurethanes with chelating carboxyl groups pendant from their backbone as magnetic recording media binders. These carboxyl groups are not formed from the copolymerization of carboxylic acid functional polyols but rather are formed from the reaction of an anhydride with a hydroxyl functional polyurethane. It is difficult to prepare a polyurethane with high carboxyl content, high macromonomer content and high molecular weight by this method because a precursor polyurethane must be prepared having a high level of free hydroxyl groups. The claimed range of such carboxyl groups is 1000 to 100,000 g/eq (0.01 to 1.0 meq/g) and in some cases they can also incorporate a vinyl polymerized macromonomer and be blended with polymers having quaternary ammonium salts.
U.S. Pat. No. 5,244,739 assigned to the assignee of the present invention, describes the use of vinyl polymeric macromonomer diols in polyurethane polymers useful in magnetic recording media. A sulfonate polar group was incorporated into some of these polymers al levels of 5000 to 30,000 g/eq (0.03 to 0.2 meq/g). Carboxyl polar groups are not taught.
Quaternary ammonium functional polymers are known to the magnetic recording media art. Examples of quaternary ammonium functional polymers include vinyl chloride copolymers, non-halogenated styrene copolymers, polyurethanes and polyethers.
Examples of quaternary ammonium functional polyurethane magnetic binders include those disclosed in U.S. Pat. No. 4,286,022 (to 3M Co.), Japanese Patent Publication No. 03/188178, 04/307420, Japanese Patent Publication No. 06/80528 and Japanese Patent Publication No. 95/19355.
Quaternary ammonium functional vinyl chloride copolymer magnetic binders are known and commercially available. Examples are described in U.S. Pat. No. 4,784,913 (to Nippon Zeon Co., Ltd.), and U.S. Pat. No. 4,861,683 (to Sekisui Chemical Company, Ltd.).
Quaternary ammonium functional nonhalogenated vinyl copolymers are described in copending U.S. patent application Ser. No. 08/054,312, assigned to the assignee of the present invention, which describes their use as magnetic binders.
Isocyanate functional polyurethane prepolymers containing oligomeric polyol segments used in magnetic recording media formulations are described in U.S. Pat. Nos. 3,150,005; 3,490,945 and 5,221,582.